Li, H.T. Jia, Q.K. Wang, L. Zhang, S.C. TUP028 FEL electron undulator cavity laser 2015 2015-12 In 2014, Xiamen University and other three research organizations received the approval to realize an infrared free electron laser (IR-FEL) for fundamental of energy chemistry. The IR-FEL covers the spectral range of 2.5-200 μm and will be built in NSRL. Two FEL oscillators driven by one Linac will be used to generate mid- infrared and far-infrared lasers. In this article we describe the design studies for the mid-infrared FEL oscillator.